Process for vanadiding metals



United States Patent 3,514,272 PROCESS FOR VANADIDING METALS Newell C.Cook, Schenectady, N.Y., assignor to General Electric Company, acorporation of New York No Drawing. Filed Nov. 10, 1966, Ser. No.593,301 Int. Cl. B23p 3/00; C13b /30 U.S. Cl. 29-194 14 Claims ABSTRACTOF THE DISCLOSURE rent density does not exceed amperes/dm. The

vanadium diffuses into the base metal to form a tight, adherent coatingcomposed of vanadium and the base metal. This process is useful forforming such compositions on the base metal.

This invention relates to a method for metalliding a base metalcomposition. More particularly, this invention is concerned with aprocess for vanadiding a base metal composition in a fused salt bath.

I have discovered that a uniform tough, adherent vanadide coating can beformed on a specific group of metals employing low current densities,that is, current densities in the range of 0.05-10 amperes/dmP.

In accordance with the process of this invention, the vanadium metal isemployed as the anode and is immersed in a fused salt bath composedessentially of a member of the class consisting of the alkali metalfluorides, mixtures thereof and mixtures of the alkali metal fluorideswith a member of the class consisting of calcium fluoride, strontiumfluoride and barium fluoride and containing from 0.01-5 mole percent ofvanadium fluoride. The cathode employed is the base metal upon whichdeposit is to be made. I have found that such a combination is anelectric cell in which an electric current is generated when anelectrical connection, which is external to the fused bath, is madebetween the base metal cathode and the vanadium anode. Under suchconditions, the vanadium dissolves in the fused salt bath and vanadiumions are discharged at the surface of the base metal cathode where theyform a deposit of vanadium which immediately diffuses into and reactswith the base metal to form a vanadide coating. In the specification andclaims I use the term vanadiding to designate any solid solution oralloy of vanadium and the base metal regardless of Whether the basemetal does or does not form an intermetallic compound with vanadium indefinite stoichiometric proportions which can be represented by achemical formula.

The rate of dissolution and deposition of the vanadium is selfregulating in that the rate of deposition is equal to the rate ofdiffusion of the vanadium into the base metal cathode. The depositionrate can be decreased by inserting some resistance in the circuit. Afaster rate can be obtained by impressing a limited amount of voltageinto the circuit to supply additional direct current.

The alkali metal fluorides which can be used in accordance with theprocess of this invention include the fluorides of lithium, sodium,potassium, rubidium and cesium. However, it is preferred to employ aneutectic mixture of sodium fluoride and lithium fluoride because somefree alkali metal is produced by a displacement reaction and potassium,rubidium and cesium are readily volatilized 3,514,272 Patented May 26,1970 with the obvious disadvantages. It is particularly pre ferred toemploy lithium fluoride as the fused salt bath in which the vanadiumfluoride is dissolved, because at the temperatures at which the cell isoperated, lithium metal is not volatilized to any appreciable extent.Mixtures of the alkali metal fluorides with calcium fluoride, strontiumfluoride and/or barium fluoride can also be employed as a fused salt inthe process of this invention.

The chemical composition of the fused salt bath is critical if goodvanadide coatings are to be obtained. The starting salt should be asanhydrous and as free of all impurities as is possible or should beeasily dried or purified by simply heating during the fusion step. Theprocess must be carried out in the substantial absence of oxygen sinceoxygen interferes with the process by forming vanadium oxide and therebypreventing a firmly adhering film of vanadium from being deposited onthe base metal cathode. Thus, for example, the process can be carriedout in an inert gas atmosphere or in a vacuum. By the term substantialabsence of oxygen it is meant that neither atmospheric oxygen nor oxidesof metals are present in the fused salt bath. The best results areobtained by starting with reagent grade salts and by carrying out theprocess under vacuum or an inert gas atmosphere, for example, in anatmosphere of argon, helium, neon, krypton or xenon.

l have sometimes found that even commercially available reagent gradesalts must be purified further in order to operate satisfactorily in myprocess. This purification can be readily done by utilizing scrap basemetal articles as the cathodes and carrying out the initial vanadidingruns with or without an additional applied voltage, thereby plating outand removing from the bath those impurities which interfere with theformation of high quality vanadide coatings.

The base metals which can be vanadided in accordance with the process ofthis invention included the metals having atomic numbers of from 24 to29, 41 to 47 and 73 to 79 inclusive. These base metals are, for example,chromium, manganese, iron, cobalt, nickel, copper, niobium, molybdenum,technetium, ruthenium, rhodium, palladium, silver, tantalum, tungsten,rhenium, osmium, iridi um, platinum and gold. Alloys of these basemetals with each other or alloys containing these base metals as themajor constituent, that is, over 50 mole percent, alloyed with othermetals as a minor constituent, that is, less than 50 mole percent, canalso be vanadided in accordance with my process, providing the meltingpoint of the resulting alloy is not lower than the temperature at whichthe fused salt bath is being o erated. It is preferred that the alloycontain at least mole percent of the base metal and even more preferred,that the alloy contain mole percent of the base metal withcorrespondingly less of the alloying constituent.

I have also found that it is advantageous to conduct the vanadidingprocess in the absence of carbon, because carbon forms a very stablevanadium carbide on the surface of the base metals thereby rendering itdifiicult to further vanadide the base metal and giving less firmlyadhering deposits. I have found that carbon can be removed from thefused salt bath by operating it as a cell, until the carbide coating isno longer formed on the surface of the base metal.

The form of the anode is not critical. For example, I can employ as theanode pure vanadium metal in the form of a rod or strip or the vanadiumcan be employed in the form of chips in a porous iron or monel basket.

In order to produce reasonably fast plating rate and to insure thediffusion of the vanadium into the base metal to form a vanadide, I havefound it desirable to operate my process at a temperature in the rangeof from about 3 800 C. to 1200 C. It is preferred to operate attemperatures of from 9001100 C.

The temperature at which the process of this invention is conducted isdependent to some extent upon the particular fused salt bath employed.Thus, for example, when temperatures as low at 800 C. are desired, aneutectic of sodium and lithium fluoride can be employed. Inasmuch as thepreferred operating range is from 900 C. to 1100 C., I prefer to employlithium fluoride as the fused salt.

When an electrical circuit is formed external to the fused salt bath byjoining the vanadium anode to the base metal cathode by means of aconductor, an electric current will flow through the circuit without anyapplied electromotive force. The anode acts by dissolving in the fusedsalt bath to produce electrons and vanadium ions. The electrons flowthrough the external circuit formed by the conductor and the vanadiumions migrate through the fused salt bath to the base metal cathode to bemetallided, where the electrons discharge the vanadium ions forming avanadide coating. The amount of current can be measured with an ammeterwhich enables one to readily calculate the amount of vanadium beingdeposited on the base metal cathode and being converted to the metallidelayer. Knowing the area of the article being plated, it is possible tocalculate the thickness of the metallide coating formed, therebypermitting accurate control of the process to obtain any desiredthickness of the vanadide layer.

Although the process operates very satisfactorily without impressing anyadditional electromotive force on the electrical circuit, I have foundit possible to apply a small voltage when it is desired to obtainconstant current densities during the reaction, and to increase thedeposition rate of the vanadium being deposited without exceeding thediffusion rate of the vanadium into the base metal cathode. Theadditional should not exceed 1.0 volt and preferably should fall between0.1 and 0.5 volt.

When it is desirable to apply additional voltage to the circuit in orderto shorten the time of operation, the total current density should notexceed amperes/dm. At current densities above 10 amperes/dm. thevanadium deposition rate exceeds the diffusion rate and the base metalcathode becomes coated with a plate of pure vanadium.

Since the diffusion rate of vanadium into the cathode article variesfrom one material to another, with temperature, and with the thicknessof the coating being formed, there is always a variation in the upperlimits of the current densities that may be employed. Therefore, thedeposition rate of the iding agent must always be adjusted so as not toexceed the diffusion rate of the iding agent into the substrate materialif high efliciency and high quality diffusion coatings are to beobtained. The maximum current density for good vanadiding is 10amperes/dmfi, when operating within the preferred temperature range ofthis disclosure. Higher current densities can sometimes be used to formcoatings with vanadium but in addition to the formation of a metallidecoating, plating of the iding agent occurs over the diffusion layer.

Very low current densities (0.010.1 amp./dm. are often employed whendiffusion rates are correspondingly low, and when very dilute surfacesolutions or very thin coatings are desired. Often the composition ofthe diffusion coating can be changed by varying the current density,producing under one condition a composition suitable for one applicationand under another condition a composition suitable for anotherapplication. Generally, however, current densities to form good qualityvanadium coatings fall between 0.5 and 5 amperes per dm. for thepreferred temperature ranges of this disclosure.

If an applied is used, the source, for example, a battery or othersource of direct current, should be connected in series with theexternal circuit so that the negative terminal is connected to theexternal circuit terminating at the base metal being vanadided and thepositive terminal is connected to the external circuit terminating atthe vanadium anode. In this way, the voltages of both sources arealegbraically additive.

As will be readily apparent to those skilled in the art, measuringinstruments such as voltmeters, ammeters, resistances, timers, etc., maybe included in the external circuit to aid in the control of theprocess.

Because the tough adherent corrosion resistant properties of thevanadide coatings are uniform over the entire treated area, the vanadidecoated metal compositions prepared by my process have a wide variety ofuses. They can be used to fabricate vessels for chemical reactions, tomake gears, bearings and other articles requiring hard, wear-resistantsurfaces. Other uses will be readily apparent to those skilled in theart as well as other modifications and variations of the presentinvention in light of the above teachings.

The following examples serve to further illustrate my invention. Allparts are by weight unless otherwise stated.

EXAMPLE 1 Lithium fluoride (11,350 grams) was charged into a monel liner(6" in diameter x 17%" deep), fitted into a mild steel pot (6%" diameterx 18" deep). The pot was fitted with a nickel plated steel flanged lidwhich contained a water channel for cooling, two ports (2% in diameter)for glass electrode towers, and two 1" port for a thermocouple probe anda gas bubbler. The whole apparatus was encased in an electrical furnacefor heating. A vacuum was pulled on the cell and the lithium fluoridemelted.

The vacuum was replaced with argon and with argon flushing through oneof the ports to prevent air from getting into the cell, vanadiumtrifluoride (47 grams) was addded to the molten lithium fluoride to makethe vanadiurn ion concentration 0.1 mole percent.

A vanadium metal anode /2" square x 6" long was immersed in the fusedsalt to a depth of 4" and with argon flowing through the cell, toprevent any oxygen from getting into the cell, a run was made inaccordance with the conditions set forth in the following table againsta nickel strip.

The sample on removal from the salt bath had a thick, black depositadhering to it which readily washed ofi? revealing a grey surface. Thesample had gained 0.111 g. as compared to a theoretical of 0.945 g.based on the reduction of V++ ions to V". X-ray emission and chemicalreagents showed the coating to contain vanadium. The coating was 0.4mil. thick, was flexible and very hard 700 Knoop hardness). Microscopicexamination showed a gradual drop in vanadium content and no sharpboundary line between the coating and the nickel base.

Seven additional runs were made employing nickel cathodes for a total of35 ampere hours at which time the nickel strips came from the cell shinyand with no adhering black deposit, indicating that interferingmaterials had now been removed from the fused salt bath. A typical runwas hard and very flexible. This coating was shown by X-ray emission tocontain vanadium and nickel and no other metals.

TABLE II Volts Current ThlS cell was run at 900 employing iron, copperand anode density gold as the cathodes and resulted in vanadide coatingspolarity amps/din 2 On these metals. Time (mins.) It will, of course, beapparent to those skilled in the 0 355; 0 art that modifications otherthan those set forth in the ,1 above examples can be employed in theprocess of this 18-832 3- invention without departing from the scopethereof. 1 0 What I claim as new and desire to secure by Letters s-88gPatent of the United States is:

' 1. A method of forming a vanadide coating on a base The sample gained0 340 gram of a theoretical 0 390 metal composition having a meltingpoint of at least 900 gram and had developed a 1.5 mil. coating that wasshiny, 15 at 93 50 mole percent of Sald base metal Campos Smooth andhard X ray examination Showed the Prey tion being at least one of themetals selected from the ence of both vanadium and nickel on the surfaceand no Class conslstmg of metals whose atfmnc numbers other elements,showing the coating to be all diifusion. to 41 F 47 and Sald {F F Table111 gives a large number of additional examples mg (l) forming anelectrlc cell contammg said metal of base metals which were vanadided inthe cell described compfmtlofl the cathoclej lolned through an externali E l 1 fter h l i k run and under an argon electrical circuit to avanadium anode and a fused salt elecatmosphere. The conditions and thetimes of reaction are tfolylewhlch 0 0 essenflally of a member of the$1358 given in the table. All yields are based on the reduction of w g fllthlum fl o m fl r m x V++ ions to metallic vanadium. thereof andmixtures of said fluorides with a member of TABLE III Current Wt.Percent Temp., Time, density, gained, coulombic Example Metal 0. min.amps/dm. grams efficiency Description of coating 1.5 mil coat; dark,smooth, ver hard outer ortion 2 Cuban; 100 15 52 210 50 l 700 KnoopHardness Numbl). p 3 .1015 C.R. steel 1, 090 45 1.0 0. 541 80 O.71 mlcoat; light grey, smooth, very hard outer portion. 4 C R Steel 1 100 035 2 67 60 {6 g8.N1l31&1I)1y,Vel'y hard surface 700 Knoop 5 Chromium 1,090 15 3. 3 0. 12 20 0.5 mil coat, grey, smooth, hard. 6... Niobium...1, 100 30 1. 8 0. 092 100 1 mil coat; shiny, smooth, hard.

1,100 90 3.1 0.103 46 coatt; lgghtkgrey smoota. m u 1 t {V 1m coa; ar,gramy,so ,p ar yapaeo 1'100 30 075 60 l adhering to a very thinditiusion layer. 1, 100 12 2.0 0. 015 50 0.3 mil coat; grey, smooth,hard. 1,000 5.0 0. 040 100 2 mil coat; silvery, bright, soft, alldiffusion. 1,000 30 1.0 0.192 80 1 mil coat; dark, grainy, hard, veryflexible. 1,000 12 1. 0 0. 017 90 0.5 mil coat; light grey, smooth,medium hard. 1,100 40 0.8 0. 215 70 0.5 mil coat; grey, grainy, hard,very flexible. 1, 090 60 0. 5 0.167 72 1 mil coat; shiny, smooth, veryhard. 1,000 30 0.7 0.065 60 0.2 mil coat; shiny, smooth. 1, 100 60 1. 00. 132 65 1 mil coat; grey, smooth, very hard. 17 Nickel 1, 000 63 0. 70. 46 93 1.5 mil coat; shiny, smooth, very hard surface.

Hours;

EXAMPLE 18 the class consisting of strontium fluoride, calcium fluo-Into a cell similar to the one described in Example 1, nde anddpanumfillonde from y f was charged a mixture (1800 grams) containing 39 moleof vana 1um fluoride, said electrolyte being malntained percent sodiumfluoride 58 mole percent potassium fluo- F a E of at least 900 l belowthe l ride and 3 mole percelit of vanadium fluoride The cell mg point ofsaid metal COII'IPOSIUOH, 1n the substantial was sealed and a vacuumdrawn on the cell and the mixed 50 i p of l (2) Contronmg the flowing 1tth n melted Ar on was allowed to flow into the 1n said electric cell sothat the current density of the Sa 1 6 cathode does not exceed 10amperes/dm. during the cell to break the vacuum and a continued flow ofargon maintained to prevent air from diffusing into the cell. formatlonof the .vanadlde coatmg and mterfuptmg A Vanadium anode Square and longwas the flow of electrical current after the desired thickness mersed 4"into the salt and the cell run against a scrap gij Vanadlde Coatmg 18formed on the base metal nickel cathode for 6 am ere hours to removeimpurities in the cell The scrap c tthode was then replaced by a The0561mm 1 Wherem the electrolyte tains calcium uori e. nickel cathodewhlle the cell was mamtamed at 900 C. and the cell run conducted inaccordance with the data The P of clam} 1 the e1ectr1yte 15 t forth inthe 011 owing table a mixture of lithium fluoride, sodium fluoride andvanase dium fluoride.

TABLE Iv 4. The process of claim 1 wherein the electrolyte 15 VoltsCurrent a mixture of lithium fluoride and vanadium fluoride. fig g; iggfi t 5. The process of claim 1 wherein the absence of oxygen isobtained by use of a vacuum. (min-)1 040 0 6. The process of claim 1WhlCh Is also conducted in -01 034 0.11 As abatterythe substantialabsence of carbon. jg'gfg 8'3; 7. The method of claim 1 wherein themetal composi- -01 010 0110 External 1.M.F. applied. tion is nickel,1818( 8 Current 0 8. The method of claim 1 wherein the metal compos1--8-8 8 tion is cobalt. 9. The method of claim 1 wherein the metalcomtungsten. The sample as removed from the bath was shiny, POSltlOn 1Ssmooth and had gained 0.142 gram of a theoretical 0.158 9: l method ofclalm 1 Wherem the metal gram. The nickel had developed a 1 mil. coatingwhich position 18 molybdenum.

11. The method of claim 1 wherein the metal composition is niobium.

12. The method of claim 1 wherein the metal composition is iron.

13. The method of claim 1 wherein the metal composition is copper.

14. A metal product produced in accordance with the process of claim 1.

554,772 3/1958 Canada.

8 OTHER REFERENCES Electrodeposition of. Coherent Deposits of RefractoryMetals, Journal of the Electrochemical Society, 1965, vol. 112-,No.-3,p. 266.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl.X.R.

